The present invention relates to a packaging material, and more particularly to a plastic packaging film prepared from an ethylene-.alpha.-olefin copolymer as a main component, which is excellent in stretchability and shrinkability.
Recently, a demand for stretchable films suitable for use for packaging foods such as meats, sea foods, vegetables, fruits and daily dishes has been increased with increase of the number and expansion of the scale of supermarkets and convenience stores.
Heretofore, as such a stretchable film, a plasticized polyvinyl chloride film has been most widely used, since the film has excellent properties such as excellent transparence and tackiness. However, the plasticized polyvinyl chloride film is disadvantageous in safety health or environmental pollution, for instance, it is easy to cause the loss of weight or the deterioration of an object to be packaged because of a large amount of steam permeating through the film, or it is easy to contaminate the object with the plasticizer, which are resulted from the use of a large amount of the plasticizer, or harmful gas, hydrogen chloride gas generates during the production of the film, melt-cutting of the film in the packaging production process, destruction of the film by fire, and the like.
Therefore, films substitutive for the plasticized polyvinyl chloride film are actively developed in an ethylene resin such as polyethylene or ethylene-vinyl acetate copolymer and polybutadiene.
Although the polyethylene or polybutadiene films have no disadvantage regarding the safety health and the environmental pollution, they are not fully satisfactory as a stretchable film.
For instance, a non-stretched low density polyethylene film causes a necking phenomenon that the stretched film has the stretched parts and the non-stretched parts, so the film thickness remarkably varies upon stretch packaging, and the elastic restoration is small, thus being not finely packaged objects. Further, the film strength is poor and the transparence is unsatisfactory. So, in order to improve the strength of the film or give the heat shrinkability to the film, it is attempted that the film is caused high orientation by biaxial stretching. However, the low density polyethylene has technical difficulties such that the film is broken during the biaxial stretching, therefore, the low density polyethylene film is prepared by means of the so-called inflation method. Thus, since the obtained film is not caused effective molecular orientation, it is inadequate in strength and shows a heat shrinkability only at temperatures close to its melting point.
Also, it has been tried to market films prepared form a crystalline 1,2-polybutadiene or ethylene-vinyl acetate copolymer as a main component, to which suitable additives such as an antifogging agent and a tackifier are added. However, since these films are prepared by the T-die method or the inflation method, the obtained films have no heat shrinkability or show the shrinkability only at temperatures close to their melting points. When packaging an object to be packaged with these films by a hand wrapper or a stretch automatic packaging machine, it is easy to break the films at an edge of a tray due to the poor film strength. That is, these films have not yet reached to a level that they can be substituted for the conventional films.
On the other hand, objects to be packaged have been diversified. Consequently, in case of packaging objects containing a large amount of water such as pickles or foods boiled down in soy, if the sealing is owing to only the tackiness of the film, the sealing joints of the film peel off due to water adhering, thus resulting in the loss of weight of the object, and further lowering of its market value. Accordingly, films capable of heat sealing have been required. Also, there is a case that, depending on objects to be packaged, tightly-fitted package cannot be obtained, if owing to only the stretchability of the film, remaining creases or looseness on the film. Accordingly, films having heat shrinkability as well as heat sealability have been required.
Also, from the viewpoints of requirements for higher productivity and speedup of packaging, automatic packaging machines such as stretch automatic packaging machines and shrink automatic packaging machines are rapidly replacing hand wrappers and are remarkably spreading, with increase of the number and expansion of the scale of supermarkets and convenience stores. Accordingly, films applicable to automatic packaging machines are required.
Further, from the standpoints of those in the wrapping art, there have been required films applicable to stretch packaging as well as shrink packaging, such that it is not required to replace by other films, even if an object to be packaged is changed, and having heat sealability.
An object of the present invention is to provide a film having excellent shrinkability as well as excellent stretchability, and a preparation process thereof.
This and other objects of the present invention will become apparent from the description hereinafter.